One of the requirements for the successful operation of any pipeline is the capability to accurately measure flow rates at many locations within the system. A number of different flow meters are currently commercially available for this purpose, each having its own advantages and limitations. Existing meters can be classified into three main types, namely obstruction meters, kinematic meters and non-intrusive meters.
Obstruction meters determine flow rate in an indirect fashion by introducing a physical obstruction directly into the flow and measuring the influence of the obstruction. For example the pressure drop across a flow restriction is often measured and correlated with the flow rate. Examples of this approach include orifice meters, venturi meters and critical flow nozzles [ref. Experimental Methods For Engineers, Fourth Edition, McGraw-Hill Book Company, J. P. Holman and W. J. Gajda, Jr., Chapter Seven]. Another example of an obstruction meter is the vortex meter, in which the obstruction causes vortex shedding. The shedding frequency is determined by means of strain sensors, thermal sensors, or pressure sensors. The shedding frequency increases with flow rate.
Obstruction meters extract energy from the flow and are therefore inherently inefficient because additional pumping capacity is required to overcome the induced pressure drop. The physical obstruction also prevents the use of pipeline-pigs for maintenance and diagnostics. Component wear can cause a shift in the discharge coefficient or shedding frequency, and therefore regular maintenance of these devices is required. Pressure or load transducers are normally mounted next to the obstruction, and therefore local power is required. For pipelines transporting flammable or explosive fluids, appropriate explosion-proof enclosures are required for these transducers. Finally, these meters have a limited turndown ratio owing to the nonlinear relationship between flow rate and pressure drop.
Kinematic type meters determine the flow rate by directly sensing the actual velocity using a turbine blade assembly that rotates kinematically with the flow. The rotational speed of the turbine is measured using a frequency pickup and is empirically related to the flow rate using an experimentally determined coefficient. These meters provide an output that is approximately proportional to volumetric flow rate and substantially independent of density. The primary disadvantages of this class of meter are the presence of moving parts, the obstruction to flow, the need for calibration, the requirement of electrical power and the physical size.
The third class of meter relies on non-intrusive methods to determine flow rate. The ultrasonic flow meter is the only meter in this category that has been commercially developed for use in high pressure natural gas pipelines. The operating principle is to compare the upstream and downstream times of flight of an acoustic pulse from one transducer to another, which are located near the inside surface of the pipe. The flow is unrestricted and therefore these devices do not produce any significant pressure drop. However, these devices require a relatively long installation length, are limited to larger pipe sizes, can suffer from acoustic noise and are sensitive to swirl in the flow.
Each of the meters described above has deficiencies in one or more of the following areas:
Size: The device should be small enough to permit installation in limited space. PA0 Low Maintenance: Moving parts should be avoided to reduce maintenance requirements. PA0 Power Supply: The device should not require electrical power at the meter. PA0 High Turn-Down: The device should provide accurate measurement of flow rates over a 50:1 turndown ratio.
Optical flow measurement offers the potential to address all of the above-noted deficiencies of "prior art" meters.
For example, flow rate may be determined by measuring the velocity of micron-sized particles suspended in a flow field. This is accomplished by determining the time-of-flight of these particles as they move between two discrete regions illuminated by laser light. This basic concept was proved by D. H. Thomson ["A Tracer Particle Fluid Velocity Meter Incorporating a Laser", Jour. of Sci. Inst. (J. Phys. E.) Series 2, Vol. 1, 929-932 (1968)] using a large gas laser, Kosters prism, two convex lenses, an imaging lens and a photomultiplier.
The time-of-flight concept has been applied to a device to measure the air speed of an aircraft (as described in United States Patents ("USP") U.S. Pat. Nos. 4,887,213; 5,046,840 and 5,313,263). Three pairs of laser sheets are projected into free air through a window located in the side of the aircraft fuselage. Small particles in the atmosphere passing through the laser sheets produce scattered light that is collected from each pair of laser sheets. This light is imaged onto photodiodes and the resulting signal is processed to determine the velocity vector. However, this prior device is designed for the aviation environment and cannot be used in pipeline applications.
Optical techniques have also been employed by numerous investigators to make measurements in laboratory environments, particularly in wind-tunnels and turbo machinery, notably R. Scholdl ["A Laser-Two-Focus (L2F) Velocimeter for Automatic Flow Vector Measurement in Rotating Components of Turbomachines", Transaction of the ASME, Vol. 102, p 412, Dec. 1980]. Additionally, UK Patent 2,295,670 describes such a configuration in which laser light from an argon ion laser is split by a Rochon prism, made parallel by a lens and then focused into two focused spots. Scattered light produced by particles passing through the two spots is imaged onto two photoelectric converters. Velocity is determined on the basis of the transit time of the particles passing between the two spots. U.S. Pat. No. 4,125,778 claims a similar device except that the relative position of the two spots could be rotated using an optical component.
A variation on the time of flight principle using laser diode arrays (i.e. multiple lasers in a monolithic device) was applied by M. Azzazy ["GRI Report 89/0201 Development of an Optical Volumetric Flow Meter" (1989)] to measure the velocity profile inside a high pressure natural gas pipe through a glass window. In this case, the image of the diode array produced a series of spots in space and the light scattered by small particles was collected and converted to an electrical signal. The frequency content of the signal and the spacing of the spots of light were then used to determine the flow velocity. Measurements were obtained at a series of locations by mechanically translating the measurement system which was located outside the pipe. The bulky size of the system, in combination with the large optical window, rendered it impractical for broad commercial applications.
Improvements to this system (i.e. using laser diode arrays) are described in U.S. Pat. No. 5,701,172 issued Dec. 23, 1997 assigned to Gas Research institute. The patent also describes the system used in combination with a hologram and a window in a pipe to produce multiple measurement locations along one pipe diameter within a pipeline. All of the illustrations and examples of the patent are limited to the case where the optical source and lens are external to the pipeline.